By Glenn Cupit This email address is being protected from spambots. You need JavaScript enabled to view it. © June, 2003

This discussion will be about RUDDER and TURN FIN dynamics on model race boats.



I would like to begin by noting that I am NOT an engineer or academic.
This is not a scientific paper. I don't offer myself as an expert by any means.
I offer no conclusions that have not been substantiated by my personal experience.
My comments are from empirical observations and my experience with model and full- scale race boats over the past 35 years. This discussion only applies to all-out race boats.
These are my OPINIONS only and as such, they are certainly open to debate and refutation by those much smarter than I.
I offer them for your consideration and contemplation to try, as you may deem prudent and necessary. If these principles don't work for you, please accept my apologies now. My intention is not to mislead anyone.


Rudder caster, refers to the forward or rearward inclination of the rudder pivot.
The term is borrowed from automotive front alignment "caster angle", which is the forward or rearward tilt of the steering upper and lower pivot points.
Positive caster is when the top of the pivot is rearward of the bottom pivot. A bicycle fork is a good example of positive caster.
Negative caster is when the top of the pivot is forward of the lower pivot or forward of vertical.
This is where the similarity with cars ends.
When discussing rudder caster angle, it is the angle between the rudder pivot centerline (with the boat at running attitude or running angle of attack) and a vertical line.
Let's call positive rudder caster the same as automotive caster whereby the top of the rudder pivot is rearward of the bottom of the pivot. (The rudder is kicked under the boat)
We will limit this discussion to straight sided, wedge rudders, one single rudder on the transom.
We will also discuss turn fins, as the rudder can at times, help or hinder the fin.
Normally, we may think that the rudder merely causes the boat to turn. This may be true, but it is not that simple! A rudder causes other effects and dynamics on both hydros and monos that must be understood in order to effectively tune or dial-in your boat.
Understanding rudder and turn fin dynamics can also help you to diagnose and correct undesirable actions of your boat.
It becomes necessary before continuing, that we discuss "drag", water "lift" and what we will call "dig-in", so that we establish a vocabulary of terms with which to describe as simple as possible in layman's terms, what occurs from rudder and fin action.
Visualize the rudder caster grossly negative with the rudder kicked back at a 45° angle. Now turn the rudder either way 90°. It should be obvious that the rudder drag will induce lift on the rudder bracket and transom. The lift will occur at the rudder pivot.
A rudder set-up like this would severely lift the transom as the rudder was turned.
If the rudder was off centerline of the transom (as it should be to get out of the prop wash or roostertail and operate in clean water), this lift would be on the right or left corner of the transom, depending on which side the rudder was located. This would cause the boat to roll to the side opposite the rudder position on the transom.
Now, let's visualize the opposite scenario – The rudder has grossly positive caster, the rudder is kicked forward 45°, under the boat, lower pivot forward of upper pivot.
It should again be obvious if the rudder was turned 90°, it would dig in like a plow! This would be severe drag and would tend to "tuck-in" or hold down the transom on the rudder side, in the turns.
The reaction of the boat from rudder drag with other than zero caster, is from the rudder (up or down, depending on + or - caster) in a straight line forward and about the boat CG (center of gravity) which is the fulcrum, or pivot point about which the boat rolls.

Now you may see why rudder placement on the inside or outside of the transom will affect the roll of the boat in a turn.
Inside or outside of the transom, refers to the turn direction of the course. In model boats, inside is the right side, as they race clockwise. In full-scale boats, inside is the left side, as they race counterclockwise. Turn direction of an oval boat race course is determined by the direction the prop revolves. As viewed from the rear model props rotate counterclockwise, full-scale boats rotate clockwise. The turn direction is an effort to neutralize the torque roll of the boat caused from the engine rotation.
Now that we see the rolling effect of a grossly + or – caster rudder, we must realize that this dynamic still occurs, but at a proportionally reduced rate, as the caster angle approaches zero. At zero caster, there is no roll effect as the rudder is turned. This may or may not be a good thing. As you may soon see, sometimes it may be desirable to induce roll from a rudder! However, zero caster is how most rudders are set-up and should be the starting point when dialing-in or tuning your race boat.

HOWEVER – if your boat exhibits undesirable characteristics, you may be able to correct and optimize them with rudder placement and small caster angles of 5° or less. Negative, or kicked back, is better than positive, or kicked forward.
It is seldom necessary to exceed 5° caster in either direction. If your boat runs well with a lot of rudder caster, there are other problems with the boat that need to be addressed. Inducing drag to fix a turning problem is not the way to excel.


Oval track race cars utilize a similar principle of one wheel affecting the diagonally opposite wheel to reduce or increase body roll in a turn. This is done by adjusting the weight on the wheels with jack screws on the coil springs.
"Jacking" a spring on a race car, increases the weight carried by that wheel and the diagonally opposite wheel. This also reduces the weight on the other two wheels an equal amount.
Car racers call this "wedge" or "cross" and is the most adjusted component used to dial-in a car at a particular track and surface.
You may have watched a NASCAR race on TV and seen a pit member "add a turn", or "take out a turn" on the jack screw over a rear wheel during a pit stop. As weight is jacked down or increased on the left rear or right front, it makes the car "tighter" by inducing understeer. As weight is jacked out of these wheels, it "loosens up" the car by inducing oversteer.
Even though "cross" or "wedge" does not apply to boats directly, the diagonal dynamic is what I want you to visualize.


Even with zero caster, the rudder will exhibit a lifting dynamic because of the drag on the rudder blade, even when straight ahead. This is not usually noticeable on hydros, but is super important on monos.
An overly deep rudder on a mono will in effect, act the same as a down trim tab in front of the rudder. This effect is more noticeable on smaller, lighter boats (.21 flat bottom monos) with no trim tabs. Mono rudders should be as short as possible and still turn tight.
In my opinion, trim tabs are a tuning crutch which is a convenient means of dialing-in a mono, but at the expense of more drag. None of my monos ever had trim tabs. I trimmed their running and turning attitude by lengthening or shortening the rudder (I had multiple identical rudders differing only in length, which could be changed quickly. In extreme cases, some bottom re-work was necessary in the last 4 inches before the transom. (This area should be dead-flat or have a slight hook of a few thousandths on the right side to flatten torque roll to the right.
To make a flat bottom mono turn fast, it must turn FLAT. Monos tend to bank themselves in a turn, which creates more drag and slows it down. Monos also tend to blow the prop out of the water in the turns, which can hook a chine and spin it out suddenly.
Positive rudder caster (rudder kicked forward under the boat) on a mono can hold the prop down in a turn, but should be tried gradually and judiciously. Usually, just a little negative or zero caster works best on monos.
Mono rudders should be on the inside (right side of transom) because we want to try to roll the boat counterclockwise in the turn to counteract the normal tendency to bank into the turn.

The turn fin on a mono should be as far back as possible without hooking the boat in a fast turn. This is a problem if you want to put the fin on the keel because the stuffing box is in the way. So, the fin must be offset to one side. WHICH side?
The intuitive choice would be the inside, or right side, which in the case of monos, is correct. To explain why, we must talk about water spray lift.


Water spray from the turn fin, prop, sponsons and transom, if it hits any part of the boat, is DRAG and usually LIFT.
If you doubt this, hold your hand out the back of a ski boat into the spray from the rudder or prop. It feels like almost the same drag if you had stuck your hand over the side into the water!
A LOT of water is thrown off the inside (left, for a model) of a turn fin in the turn. It is extremely important to realize this and know that wherever the water hits the boat or fin bracket, it is causing drag and lift at that point on the boat.
Normally, we don't want this lift, but in certain instances it can and has been used to serve a purpose. We for sure don't want to gain any lift in the turn fin area of an outrigger with the fin mounted on the inside sponson! This could easily blow the fin out in the turn!
However, consider the case of a conventional hydro - - say an old Hughey. Ed Hughey mounted the fin under the tunnel, on the left or outside! Intuitiveness, said that would not work, EVERYONE had the fins on the INSIDE, or right.
Well - - what happened was, it not only worked fine, it caused these boats to turn FLAT and tight at wide open throttle! The fins were not canted or angled and they did not unhook! What I think was happening was the spray off the left side of the fin hit the bottom of the tunnel on the outside (left) of the hull and caused lift at that point which offset the tendency to roll counterclockwise in a tight turn! This made the Hugheys turn tight and flat.
So, if the turn fin is mounted under the center section of an outrigger, it need not be canted and should be on the LEFT side! The problem with doing this on an outrigger, is that the center section is not wide enough to allow the fin to be offset very much until the spray would completely miss the bottom and blow out the left side, eliminating the desired lift.
On a mono, however, we want the opposite roll, counterclockwise. We want to reduce the tendency to bank into the turn, so we want the turn fin spray to lift the RIGHT side of a mono.
This explains the above correct choice of placing a mono turn fin on the right side.


OK – now we come to rudder placement on hydros. Inside or outside?
For years, the only choice was the intuitive one – the inside or right side of the transom. However, in the model boats of the mid-seventies and some of the full-scale hydros of the eighties, we began to see outside rudder placement! This works very well for the following reasons:
Remember the earlier discussion about cross dynamics from the rudder with caster other than zero? A rudder on the outside with slight negative caster or kick-back, PLANTS THE INSIDE SPONSON AND FIN in the turn!
The only problem with this set-up, is the clockwise roll force thus imparted can aid the prop torque lift of the left sponson when shutting down on turn entry and the boat may fly and blow-over with a clockwise roll just into the turn. This can usually be dialed-in by reducing the negative caster slightly.
Too much negative caster also causes the prop to blow out in the turns and the boat will "hop" through the turn.
We are talking about fine adjustments with feeler gauge stock under the top or bottom of the rudder bracket and the transom. I made thin wedges of aluminum in ½° increments from ½° to 5°. Once the boat was dialed-in, I would mill the bracket on that angle and eliminate the shims. I don't ever remember going back and changing the caster once I felt it was dialed-in. We tuned the full-scale Shazam and the Unlimiteds exactly the same way. It does not take much caster! This was a closely guarded secret back then and I have never heard this discussed publicly.


Outdrives and steerable struts are "rudders with thrust", and should be included in this discussion.
Caster also applies to outdrives, but the caster dynamics are opposite that of rudders.
Because these "rudders" are thrusting instead of dragging, positive and negative caster of outdrives causes boat roll OPPOSITE that of rudders discussed above.
The dynamic is the same, just opposite effect.
Negative outdrive caster, unlike negative rudder caster, causes the nose to lift in the turn (to either direction). Positive outdrive caster applies down pressure to the nose of the boat in a turn.
Since outdrives are in the center of the transom, no diagonal dynamic is transferred to the front corners as with a castered rudder. The roll effect comes from the up or down thrust angle in the turn and the natural tendency of a mono to bank into the turn.
Don't confuse outdrive caster with thrust angle straight ahead. You can have whatever outdrive thrust angle you wish. Caster is independent of straight ahead thrust. Remember, caster refers to the PIVOT angle of the outdrive and has nothing to do with the straight ahead thrust angle of the prop shaft.
Outdrives have not proven effective on hydros because there is no rudder blade area, only a small fin under the prop. The rudder blade area in the water of a hydro is necessary to keep the transom tracking with the boat centerline in the straightaway.


A surfacing prop is trying to "walk" across the water as the lower portion of the blade enters the water. In a model, this "walk" force is to the left and forces the transom left causing the boat to pull right, requiring left rudder to keep the boat straight.
High pitch, high lift, high rake and inefficient props, prop walk more noticeably.
Usually, prop walk can be controlled and kept to a minimum by using a deeper rudder. It takes a lot of side force to "drag" a rudder sideways through the water on the straightaway.
Surface props must be razor sharp and have equal pitch and contour on all blades.
So, without rudders, outdrives will tend to propwalk if used on hydros.


This has nothing to do with rudders and fins, but since we were discussing propwalk and straightaway "pull", I thought I would mention hydro "asymmetry".
Asymmetry is when the hydro is not symmetrical, right and left of the centerline of the center section.
Most asymmetry is an attempt to eliminate prop walk pull and keep the inside sponson down in the turn.
This asymmetry involves offset sponsons, different sponson angles right to left, engine offset, angled and/or offset drive shaft or strut, weighted right sponson, etc.
I'll stop here and let you ponder the effects of the above asymmetry, the effects of which you may not have realized about your boat.


I think that a turn fin should ideally be vertical with the water. I say this admitting that I ran bent or angled fins in model boats and canted fins in full-scale hydros.
I didn't completely understand all that goes on with turn fins and rudder dynamics when I was running model boats. I came to gain a better understanding only after experience with model boats and full-scale hydros.
I now feel that bent turn fins are just another easy tuning crutch. It's an important crutch in that it can quickly correct unhooking turn fins in high G turns, by minor alignment changes.
Bent, canted or angled (same thing) turn fins MUST be very accurately aligned or they will lift or dig-in and slow down the boat drastically in the turns. They must have some means of adjustment and be able to remain rigid at the forces applied to it in the turn.
The fin should be of a material that will be rigid with the thinnest material, even if it must be heavier! Going to a lighter, but thicker material is kidding yourself. The very slight weight saving is far overshadowed by the increased drag of the thicker material!
I now feel that 1/16, 5/64, 3/32 or 1/8 hardenable tool steel is probably the best. Ed Kalfus knew this 40 years ago. All his rudders and fins were hardened & blued tool steel, very thin - - and WE thought we knew better!
The neat thing about angled turn fins is that you can tweak them by pivoting them on one mounting hole, the other one slotted, to make the boat turn lighter or heavier on the right sponson. But, all you are doing is inducing drag (lift or dig-in), which can be usually dialed-in with other adjustments, one of which is rudder caster.
However, angled turn fins are aligned optimally at only one angle of attack. That means if it is dialed-in in the turns when the transom is lowered; then in the straights as the prop lifts the transom to the maximum, it is NOT dialed-in and is either lifting or digging-in.
I suggest that you don't follow the pack and try for yourself a straight or SLIGHTLY canted fin along with a left rudder with a little negative caster. Start with a vertical fin. Don't angle it unless it blows out. If it blows out, decrease the rudder caster until you feel the increased rudder drag has gone too far. THEN, cant the rudder, A LITTLE!
Optimum fore-aft turn fin location will vary slightly for different hydros. If you make provision for easily adjusting the fin location when building your hull, you will be pleased you did so when optimizing your racing set-up during testing.

Never run a bent turn fin on a timed straightaway run. Put on a shallow, vertical fin. You may have to run a deeper rudder because the straightaway prop will usually want to propwalk more than an oval prop.
Remember, our comments about turn fin spray. Design the placement and bracketry so that no spray induced lift occurs.
Outrigger turn fin placement usually necessitates bolting the fin to the inside of the right sponson, trailing the fin back and down to get it close to the boat CG. If you trim the top of the fin down close to the water line, it makes it more subject to twisting in the turn. If you leave it tall all the way back, the spray off the inside bottom of the sponson causes more unwanted drag on the outside of the fin. A happy medium compromise must be determined between flexing and drag. Again, a rigid material fin will really help with the flexing here.
To check if your fin is strong enough, consider this: If your boat weighs 8 pounds, and negotiates a turn between 3 and 4G's (I think this is a good guesstimate. Let's hear from the engineers.), then the force on the fin is 24 to 32 pounds, if it is on or close to the CG.
Make a block of wood to bolt your fin to just like it is mounted on your sponson. Mount the block in a vise with the fin horizontal. Mark the waterline on the fin. Mount a wire pointer to indicate the vertical position of the fin in the fixture. Place a 24 pound weight on the portion of the fin below the water line. It may be easier to make a wood clamp for the fin from which to hang the weight. I bet you will be surprised at the deflection of your stainless steel fin! Now, try a 30 pound weight! These weights may be incorrect. Maybe our engineers will suggest appropriate weights, if I am off the mark. Also, consider the force on the fin if your boat weighs 10 pounds!
I think you will find that your bent, stainless fin is bending severely in the turns. That's why the stainless fins must be bent so much; because they flex and if not bent severely, become less than vertical under the above G forces.
If the above proves true for you, go make a straight hardened carbon steel fin, sharpen, polish and test it as above. If it is thick enough, it will be rigid. Now have it plated and mount it slightly canted.


A straight edge, parallel to the water line, held against the sides of a non-wedge fin, should be parallel to the boat centerline as viewed from the top. To properly align a wedge turn fin, the wedge angle must be bisected and the resulting centerline aligned with the boat centerline. A wedge shim, ½ the angle of the included angle of the wedge fin can be made as an alignment tool. This shim is then held between the straight edge and the fin when aligning as above.
Of course, with a bent or canted fin, the portion of the fin to be aligned is all portions in the water.
You never want to purposely misalign a fin. More drag. You only need ONE rudder!
You should not need to cant or angle a straight fin more than a degree or so more than the dihedral angle of the sponson.
Again, vertical or slightly canted, straight fins are much easier to align and do not induce as much drag as do severely bent fins, when the running attitude changes from the turns to the straights.


My intent here was to stimulate your thoughts and open your mind to possible alternate approaches to tuning your boat.
Most boat racers don't think about some of the concepts discussed above.
If I have contributed one "Ah-Ha" to your understanding of rudder, fin and spray dynamics - - I am very happy.

If none of this works for you, I am sad.

This article may be freely shared, reprinted or published with no further permission necessary from me, other than advising me by e-mail, if and where published.
This permission is granted ONLY, if copied in it's entirety, with no editing.
(Reprints must include the beginning and ending copyright acknowledgements along with this notice)
I will be happy to send this article upon request as an e-mail attachment, for ease of printing.
Glenn Cupit - P O Box 113 - Wilson, LA 70789 This email address is being protected from spambots. You need JavaScript enabled to view it. © June, 2003



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